PLEASANTON, Calif. -- The 30-foot-long RV stood parked on Main Street in this sunny, small town, taking up much of a city block. The Defenders of Wildlife Action Fund had festooned the vehicle with colorful caricatures of House Resources Committee Chairman Richard W. Pombo (R-Calif.), along with catchy slogans such as "Honk if you're tired of Richard Pombo wasting your money."

In a state where nearly all 53 House members are assured reelection, Pombo is facing a GOP primary challenge and a serious general election contest. Though he enjoys the advantages of incumbency, a Republican-leaning district and sizable campaign coffers, the 14-year congressional veteran is engaged in his first genuine race in years because of a concerted assault by national environmental groups.

It remains unclear whether the groups will succeed. The district has a 7 percent Republican edge. And with more than $1 million, Pombo's war chest dwarfs those of his opponents. Many voters care about the environment, but they rarely consider it the deciding issue when picking a candidate.

Environmentalists contend that they have a chance to win here, mainly by tying Pombo's legislative actions to the broader theme of a corrupt Republican-led Congress. A slew of organizations, including Clean Water Action and the Sierra Club, have joined Defenders of Wildlife Action Fund in attacking Pombo's record on endangered species, timber logging and stewardship of national parks. They have knocked on thousands of doors, sponsored radio ads and called voters in Pombo's district in an effort to torpedo his political career.

"Clearly, he is our number one priority, in terms of letting his constituents know exactly how aggressive he's being here," said Rodger Schlickeisen, chief executive of Defenders of Wildlife Action Fund.

The group, which is the political arm of the Defenders of Wildlife nonprofit organization, recently named Pombo its first "Wildlife Villain of the Year." It has a Web site called "Pombo's in Their Pocket" linking the congressman to the corporate interests that he oversees.

Pombo, who has championed an overhaul of the Endangered Species Act as well as offshore oil drilling and gas exploration in the Arctic National Wildlife Refuge, said he is not surprised that he has become such a target of environmentalists.

"I don't think any one of them would honestly say they can beat me," he said in a recent interview at his cattle ranch in his home town of Tracy. "They will raise a ton of money by trying to make me the boogeyman."

Pombo acknowledged that this year will be tougher than 2004, when he faced a write-in Democratic candidate who garnered less than 40 percent of the vote. Since 2003, 75,000 new voters have moved into his district, many of them more liberal-leaning voters who left the San Francisco area in search of cheaper real estate.

"They're used to a different kind of Republican than they're getting from Pombo," said Sierra Club Executive Director Carl Pope, whose group has knocked on 15,000 front doors in the district and has gathered thousands of names on a petition criticizing Pombo's record.

Pombo's district covers the more rural area around Tracy and part of the East Bay where San Francisco and Silicon Valley commuters reside. Though environmentalists are targeting both areas, they are more optimistic about winning over East Bay voters.

"This is a district that's clearly in the realm of a Democrat," said Defenders of Wildlife Action Fund Political Director Mark P. Longabaugh, who noted that liberal Sen. Barbara Boxer (D-Calif.) won the district by three points in her last election. "There's a big difference between a Republican-leaning district and an overwhelmingly Republican district."